With the advent of pollution control laws, a desire and need for an efficient, long lasting furnace exhaust duct system has become crucially important, especially in the field of steel manufacturing. Exhaust fumes, particularly from steel making furnaces, such as the Bessemer converters and electric arc furnaces, contain a large amount of very hot pollutant and combustible contaminants which must be removed from the exhaust gases before they can be exhausted to the atmosphere. The steel making furnaces, since they must operate at a temperature substantially higher than that required to melt the steel, emit exhaust gases at relatively high temperatures which damage and rapidly deteriorate conventional duct work either through oxidation or actual melting and rupturing of the duct work. Several types of fluid cooled exhaust duct work systems have been proposed to eliminate this failure problem but many if not all of such proposals have been found to be unsatisfactory and extremely costly. Failure is often caused by a non-uniform thermal expansion of the inside and outside wall of some of the water cooled, double wall type exhaust ducts and the use of expansion joints to solve this problem has proven unsatisfactory.